Think of the photograph as identity data with a visual presentation layer. The picture must remain recognisable while the technical delivery matches the receiving system.
The measurement first
Millimetres and inches describe physical output; pixels describe digital dimensions; DPI connects pixels to physical size during printing. If an authority already states the required pixel dimensions, use them directly. If you are building a print file from millimetres, the basic relationship is pixels = millimetres × DPI ÷ 25.4.
Where calculations stop being enough
A mathematically correct file can still fail because the face is the wrong size inside the frame, the background is unsuitable, the compression introduces visible artefacts, or the application expects a different file type. Finish every conversion with a visual and application-specific check.
For a local print
Measure one printed copy before producing the whole sheet. If the dimensions are wrong, fix the print scaling first rather than resizing the source photograph unnecessarily.
Passport-size is a family of workflows
Travellers often prepare photos for several destinations. Treat each destination as its own output even when one original portrait is used for all of them.
This keeps country-specific rules visible and prevents accidental submission of the wrong crop.
A note for this exact search
The safest reuse pattern is master first, derivative second. Never make a new country-specific output by repeatedly editing the last JPEG you exported.
How to Convert Passport Photo Pixels to Inches
Use the Studio after you have confirmed the requirement: upload or capture, prepare the image, then export the document-specific result.